[RFC]-Approaches to user space probes

[Stone, Joshua I]

Stone, Joshua I wrote:
> Keshavamurthy Anil S wrote:
>> [...]
>> If in the user program, strlen() is called only once and
>> assuming that the return probe failed to initialize due to
>> no kretprobe instance, your return probe handler will
>> never gets fired and your hence in_strlen[tid()] will
>> never gets reset and  the pagefault count will have
>> wrong value.
> 
> Well, in a world where kretprobes can miss, the best you can do is a
> "statistical" sampling.  If an event only heppens once, it may not be
> of much interest anyway.  (It would be nice to see this limitation of
> kretprobes go away, but I haven't reviewed the code enough to know why
> that limitation is necessary...)

Sorry, I didn't quite understand your point.  If only one strlen is
called and then missed on return, then *every* pagefault in that thread
from then on will be attributed to strlen.  I agree this is a problem
with my example - and also a problem that may crop up in SystemTap
scripts in general.  All the more reason why I wish kretprobes were more
"robust"...

Josh